Justin Kaplan, a Pulitzer Prize-winning biographer and mainstay of Cambridge and Truro literary life, died Sunday at Mount Auburn Hospital. He was 88. The cause of death was complications from Parkinson’s disease, according to his daughter Susanna Donohoe.

Mr. Kaplan won both a Pulitzer and a National Book Award for his biography of Mark Twain, “Mr. Clemens and Mark Twain” (1966), and an American Book Award for “Walt Whitman: A Life” (1980). He also served as the editor of the 16th and 17th editions of “Bartlett’s Familiar Quotations.”

A gregarious man, as sociable as he was social, Mr. Kaplan was universally known as “Joe.” He was a much-loved literary citizen — “both on the page and in person,” the novelist James Carroll said in a telephone interview. “He had an unusual combination of gravity and wit. He presented himself very modestly, yet he was clearly the most brilliant person in the room.”

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With his wife, the novelist Anne Bernays, Mr. Kaplan was the center of a literary social circle at the heart of 02138, the Harvard Square ZIP code. The Kaplans, who moved from New York to Cambridge in 1959, lived for many years in a 16-room house on Francis Avenue. Among such neighbors and friends as Julia Child and the Harvard economist John Kenneth Galbraith, the Kaplans fit right in. It was a mark of the couple’s closeness — “Anne and Joe” was a phrase so commonly heard it came to seem like one name — that they collaborated on not one but two books, “The Language of Names” (1997) and a memoir of growing up and coming of age, “Back Then: Two Lives in 1950s New York” (2002). Their 60th anniversary would have been this July.

“They really were a pair,” Daniel Okrent said in a telephone interview. Okrent, an author and the first public editor of The New York Times, was one of Mr. Kaplan’s many friends from the Outer Cape, where the Kaplans built a second house in 1973. “I’ve never known anybody who was so good natured. He did not strut — the way Pulitzer winners strut! He was always interested in what you had to say — even though it was a safe bet that he knew more about what you were saying than you did. Good joke teller, good joke listener, too — and that’s a rare combination.”

Mr. Kaplan belonged to a small but distinguished authorial fraternity: the non-academic literary biographer. As the critic Alfred Kazin once wrote, Mr. Kaplan exemplified “the kind of historical intelligence that a biographer needs most.” Although he did teach on occasion — such as at the College of the Holy Cross in Worcester during the early 1990s — he was in essence an unaffiliated man of letters, like Van Wyck Brooks before him, or Blake Bailey today. Mr. Kaplan cared as much about his readers as he did about his subject. Harvard professor Helen Vendler, reviewing Mr. Kaplan’s Whitman biography in The New York Times Book Review, called him “a born narrator.”

“When I read a conventional biography I always get hung up on Chapter Two,” Mr. Kaplan said in a 1981 Globe interview. “For years I assumed the shape of a literary biography must imitate the shape of the subject’s life. Then I realized it was more significant to recreate the life’s density, texture, and meaning.”

So the Twain biography begins when its subject was 29 — and the Whitman biography at the end of the poet’s life. “This afforded an opportunity to bring out his powerful thematic preoccupations,” Mr. Kaplan continued, “his sense of existence as an unbroken cycle of renewal, birth to death to rebirth — which is the way he thought of ‘Leaves of Grass.’ ”

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The son of Tobias D. Kaplan, a shirt manufacturer, and Anna (Rudman) Kaplan, a homemaker, Mr. Kaplan was born in Manhattan, on Sept. 5, 1925. Both of his parents had died by the time he was 9. “I spent a lot of time as a boy playing in Central Park and walking around Manhattan by myself,” he recalled in that Globe interview. He was raised by an older brother and the family’s West Indian housekeeper.

Mr. Kaplan would spend much time with her in the kitchen, a fact whose significance would emerge after he and Bernays married. Mr. Kaplan was the experienced cook and food shopper, not Bernays, a self-described “domestic illiterate.”

At his brother’s urging, Mr. Kaplan went to Harvard. He majored in English, getting his degree in 1944. He did graduate work in English for two years. Increasingly dissatisfied with academic life, he moved to New Mexico. Being out there furthered his education as Harvard had not. “The openness and the beauty of the Southwest,” he said in that 1981 interview, “made me aware of American writers in a way I had never considered before.”

Returning to New York, Mr. Kaplan went to work in publishing. He spent eight years as a freelance book editor before becoming a senior editor at Simon & Schuster. The “house intellectual” there, as Mr. Kaplan once put it, he numbered among his authors the philosopher Bertrand Russell, the novelist Nikos Kazantzakis, and the sociologist C. Wright Mills.

In “Back Then,” Mr. Kaplan compared Simon & Schuster to “a summer camp for intellectually hyperactive children.” The camp did not have a curfew. He wrote in the memoir of dancing at a party with Marilyn Monroe, “gently kneading the little tire of baby fat around her waist.”

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The camp also looked out for its campers. When Mr. Kaplan decided to leave Simon & Schuster to write his Twain biography, the firm signed up the book — and let Mr. Kaplan write the contract. The book was what brought the Kaplans to Cambridge, which offered the double attraction of comparative calm and access to Harvard’s Widener Library. Over the next seven years, Mr. Kaplan researched and wrote the biography. Few first-time authors meet with such success — or have cause to suffer such professional anxiety. “I couldn’t sleep,” Mr. Kaplan recalled in 1981. “I had nightmares. I broke out with a skin disease. ‘You’ve thrown over a terrific job in New York,’ I told myself. ‘What do you think you’re doing?’ ”

After “Mr. Clemens and Mark Twain,” Mr. Kaplan published “Mark Twain and His World,” an illustrated book on the author, and a biography of the muckraking journalist Lincoln Steffens (both 1974).

Mr. Kaplan embarked on two subsequent biographies, of Ulysses S. Grant and Charlie Chaplin. Neither worked out. Instead, Mr. Kaplan took on the editorship of Bartlett’s. The job was ideally suited to Mr. Kaplan’s blend of erudition, gusto, and curiosity. “It’s every writer’s dream,” he said in a 1990 Globe interview. “Every day, I look over my shoulder because I have the sense people think I’m goofing off.”

“You can’t do it systematically,” Mr. Kaplan explained. “You do it associatively. One thing reminds you of another thing. You have to see whether it is not only quotable, but whether it has been quoted. I’m not doing an anthology of literary gems, but trying to find out what people have been quoting, what is stuck in their minds.”

Mr. Kaplan published his final book, “When the Astors Owned New York: Bluebloods and Grand Hotels in a Gilded Age,” in 2006. He belonged to the Massachusetts Historical Society, the American Academy of Arts and Sciences, and the American Academy of Arts and Letters, but the membership he most enthusiastically held may have been in the informal academy he and his wife sustained for so many years.

“If there’s a writer’s community in Boston,” Carroll said Monday, “they established it. There was a period of about 15 years when their house was the center of the writing life in Boston. Joe was the pillar, and Anne was the flame. Between the two of them they made a big difference in the life of the city.”

In addition to his wife and daughter, of Ashland, Mr. Kaplan leaves two other daughters, Hester of Providence and Polly of Rhinebeck, N.Y.; and six grandchildren. A celebration of his life is planned for the spring.

Joseph P. Kahn of the Globe staff contributed to this obituary. Mark Feeney can be reached at mfeeney@globe.com.